1. Field of the Invention
The invention primarily concerns restorative dentistry, especially methods and devices useful for making out-of-the-mouth or extra-oral dental restorations from dental restorative or composite resin materials. The invention also concerns a model or die from which a prosthetic dental restoration can be made and a method of making such a model or die. The invention further concerns models or dies useful for such purposes as the restoration of art objects such as statues.
2. Description of Related Art
In spite of remarkable technological advances in prosthetic dental restorative or composite resin materials, amalgams typically are easier to install, can be completed in a single visit, and are regarded by many practitioners as having superior durability. For such reasons, the amalgams continue to predominate in posterior dental restorations in spite of their toxicity, aesthetically undesirable color, and the usual need to remove healthy portions of a tooth in order to interlock the amalgam into a cavity. Dental restorative or composite resins also can be applied in a single visit by being tamped into a cavity, shaped or sculptured, then cured by exposure to light and finished with a bur. The step of shaping or sculpturing before curing is cumbersome, as is grinding after curing. Furthermore, shrinkage of the resin during curing produces strain on the tooth and can result in marginal leakage. Even when shrinkage is minimized by incremental curing and the dentist has sufficient skill to sculpture the uncured resin to duplicate the original tooth contour precisely, the procedure is sufficiently demanding and time consuming that the dentist may prefer the convenience of an amalgam.
It has been suggested that the effect of resin shrinkage can be minimized by using a model or die to form an extra-oral prosthetic dental restoration such as an inlay. Such a model can be formed from dental or gypsum stone (Plaster of Paris), from thermoplastic resin as in U.S. Pat. No. 2,135,404 (Wheeler), or from epoxy resin as illustrated in Jensen et al., "Polymerization Shrinkage and Microleakage," a paper published in Posterior Composite Resin Dental Restorative Materials edited by Vanherle et al., pages 243-262 (Peter Szulc Publishing Co., The Netherlands, 1985). The Jensen article in a table at page 258 lists advantages and disadvantages of each of in-the-mouth and out-of-the-mouth "`inlay` posterior composites," the advantages of the latter being:
"Reduced stress on cusps from polymerization shrinkage
Better marginal adaptation at gingivo-proximal (no overhang)
Control of proximal contacts
Better contours and anatomy
Easier to obtain a better surface finish
Possible increased abrasion resistance because resin can be heat cured under vacuum"
Among the listed disadvantages of the out-of-the-mouth or extra-oral "`inlay` posterior composite" are that normally more than one dental appointment is required, thus requiring a temporary restoration, and that there is increased cost due to laboratory procedures.
While the Jensen article refers to the use of an epoxy die for molding dental restorations, and the Wheeler patent refers to the use of dental patterns made from certain thermoplastic resins, such dies or patterns are more commonly made from dental or gypsum stone. Gypsum stone is generally regarded as the state of the art molding material against which other materials are measured. The thermoplastic resins of the Wheeler patent are said to be grindable but must be melted at fairly high temperatures. This can cause unacceptable shrinkage of the model and poor restoration fit. Like epoxy resin, gypsum stone takes a long time to harden, thus requiring two visits to the dentist and a temporary restoration between visits. The need for two visits can be exceedingly inconvenient to patients who live in remote areas, and the need for a dentist to use a dental laboratory can be troublesome when the closest laboratory is at a distant location.